(1) Field of the Invention
The present invention pertains to the construction and method of constructing a powdered metal sintered bearing having a polygonal interior bore that improves its ability to provide a flow of oil to the bore.
(2) Description of the Related Art
Metal-powder-sintered bearings or powdered metal sintered bearings are porous bearings that can operate for long periods of time without being resupplied with additional lubrication. An example of such a bearing 12 is shown in FIGS. 1 and 2. Referring to FIG. 2, the bearing shown is a spherical sintered bearing with its exterior surface 14 having a generally spherical configuration. A cylindrical center bore 16 passes through the bearing and supports a motor shaft for relative rotation therein. A cross-section of the bearing 12 is shown in an operative environment in FIG. 1. The bearing is retained in a two part circular enclosure 18 that also contains a lubricant absorbent wicking material 22. In many applications, the wicking material 22 is exposed to a lubricant reservoir of the electrical device in which the bearing 12 is employed through an opening provided in the enclosure 18. The wicking material 22 absorbs the lubricant and thus provides a source of lubricant around the entire exterior surface 14 of the bearing. The powdered metal sintered bearing 12 is porous and the lubricant contained in the wicking material 22 passes through the bearing from its exterior surface 14 through to the center bore 16 where it provides lubricant to the rotating shaft supported by the bearing. The operative environment shown in FIG. 1 is only one general example of how powdered metal sintered bearings are used and many other use environments also exist. This type of bearing is particularly useful in applications where it may be inaccessible or inconveniently accessed making it impossible or in the least difficult to lubricate the bearing. The bearings are often referred to as sliding bearings and also spherical bearings. They are primarily used in applications where they will experience light to medium loads and where a bearing of relatively small size is needed. For example, they are often used in electric motors that operate household appliances such as dishwashers, clothes washers and dryers, etc.
As suggested by their name, the bearings are made by compressing mixtures of metals such as copper and tin, or graphite, iron, or teflon. The mixtures are compressed into the desired form of the bearing and then the form is sintered in a reducing atmosphere (or increasing pressure) forming the powdered metal into a coherent mass by the heating without melting the powdered metal. The process forms a porous metal bearing. The porosity of the bearing allows the lubricant to be conveyed through the bearing from its exterior surface to its interior bore that supports a rotating shaft of an electric motor.
As is known in manufacturing powdered metal bearings, the greater the compacting force, the stronger the bearing formed by the powdered metal. The greater compacting force applied to the powdered metal in the molding process gives the bearing formed greater strength to resist wear and fragmentation of the powdered metal in use. However, the more compacting force that is applied to the powdered metal in the molding process decreases the porosity of the bearing formed. Thus, producing a stronger bearing by increasing the compacting force has the disadvantage of reducing the porosity of the bearing and thus reducing its ability to convey lubricant from its exterior through to the interior bore of the bearing.
Powdered metal sintered bearings are also limited in their areas of use due to their construction. These types of bearings have been known to fail when subjected to unbalanced loads or too high of a load. Powdered metal bearings experience failures when subjected to unbalanced load conditions because the loading of one area of the bearing's interior bore tends to close off the bearing porosity in that unbalanced loaded area and starves the journal or bore of oil in this area due to the closing of the pores. With the closing of the pores, lubricant is prevented from reaching the unbalanced loaded area of the bore causing metal to metal contact between the bearing bore and the rotating shaft it supports which increases wear and temperature which could ultimately lead to the failure of the bearing. It is necessary that the entire perimeter or interior circumference of the bearing interior bore remain porous and permeable to supply oil to the journal. When the load on the bearing is too high, the pressure exerted by the motor shaft on the loaded area of the bearing interior bore forces oil back into the porosity of the bearing in the loaded area allowing metal to metal contact which increases wear and temperature and could ultimately lead to the failure of the bearing.
What is needed to overcome the problems experienced with prior art powdered metal sintered bearings is a powdered metal sintered bearing having areas of its interior bore and its exterior surface that are compacted for strength to provide load bearing surfaces that resist wear and fragmentation of the powdered metal in use of the bearing, and also has areas in its interior bore and on its exterior surface of greater porosity or less density of the powdered metal providing a porous path for conveying lubricant from the exterior surface of the bearing to the bearing interior bore.